The dehydrogenation of hydrocarbons is an important commercial hydrocarbon conversion process because of the existing and growing demand for dehydrogenated hydrocarbons for the manufacture of various chemical products such as detergents, high octane gasolines, oxygenated gasoline blending components, pharmaceutical products, plastics, synthetic rubbers, and other products which are well known to those skilled in the art. One example is the dehydrogenation of propane to propylene which is one of the most important raw materials in the petrochemical industry. Another example of this process is the dehydrogenation of isobutane to produce isobutylene which can be polymerized to provide tackifying agents for adhesives, viscosity-index additives for motor oils, and impact-resistant and antioxidant additives for plastics. Another example of the growing demand for isobutylene is the production of oxygen-containing gasoline blending components which are being mandated by the government in order to reduce air pollution from automotive emissions.
Those skilled in the art of hydrocarbon conversion processing are well versed in the production of olefins by means of catalytic dehydrogenation of paraffinic hydrocarbons. In addition, many patents have issued which teach and discuss the dehydrogenation of hydrocarbons in general. For example, U.S. Pat. No. 4,430,517 (Imai et al), U.S. Pat. No. 4,438,288 (Imai et al), and U.S. Pat. No. 6,756,340 (Voskoboynikov et al.) discuss a dehydrogenation process and catalyst for use therein.
The dehydrogenation of hydrocarbons process utilizes radial flow reactors constructed such that the reactor has an annular structure and annular distribution and collection devices. The devices for distribution and collection incorporate some type of screened surface. The screened surface is for holding catalyst beds in place and for aiding in the distribution of pressure over the surface of the reactor to facilitate radial flow through the reactor bed. The screen can be a mesh, either wire or other material, or a punched plate. For a moving bed, the screen or mesh provides a barrier to prevent the loss of solid catalyst particles while allowing fluid to flow through the bed. Solid catalyst particles are added at the top, flow through the apparatus, and are removed at the bottom, while passing through a screened-in enclosure that permits the flow of fluid over the catalyst. For example, the screens are described in U.S. Pat. Nos. 9,266,079 and 9,433,909 (Vetter et al.).
The reactor 10 in FIG. 1, includes a reactor shell 20, one partition in the form of a centerpipe 30, an outer partition in the form of screened partition 40, and a solid particle, or catalyst, bed 50. The reactor 10 can be configured so that fluid enters the reactor 10 through an inlet 32 at the bottom of the reactor and travels upwardly through the centerpipe 30 in the direction indicated by arrow 11. As the fluid flows upwardly, portions of the fluid are directed radially through the centerpipe into the catalyst bed 50 where the fluid contacts the catalyst and reacts to form a product stream. The product stream flows radially outwardly through the outer screened partition 40 and into the annular space 14 between the screened partition 40 and the reactor shell 20. The product stream is collected in the annular space 14 and passes through a reactor outlet 12.
The reactor, according to FIG. 2, may be configured to have an opposite flow pattern such that fluid enters through an inlet 13 and enters the annular space 14 between the reactor shell 20 and the outer screened partition 40 and flows radially inwardly through the catalyst bed 50 where it contacts the catalyst and reacts to form a product stream. The product stream flows radially inwardly through the center pipe 30 where it is collected in the centerpipe and exits through the outlet 33.
If the reactor includes a radial outward flow configuration like that shown in FIG. 1, the centerpipe 30 includes an outer catalyst-side profile wire screen and an inner fluid-side perforated plate. The outer partition may also include an inner catalyst-side profile wire screen and/or an outer fluid-side perforated plate. Alternatively, where the reactor includes the radially inward flow configuration of FIG. 2, the outer partition 40 includes an inner catalyst-side profile wire screen and an outer fluid-side perforated plate. The centerpipe 30 may also include an outer catalyst-side profile wire screen and/or inner fluid-side perforated plate.
The centerpipe 30 and partition 40 must perform the duty of preventing the passage of solid catalyst particles and allowing the passage of fluid, while providing structural strength to hold the catalyst against the pressure of the weight of the solid particles.
In radial bed reactors with substantially continuous catalyst circulation, the forces exerted on the catalyst bed by the gas flow must be considered to ensure uninhibited catalyst movement. The direction of the gas flow through the catalyst bed is generally cross current to the desired direction of the catalyst movement in the active bed. Under the right conditions, excessive gas velocities may impact catalyst movement either by holding up solids flow, i.e., “pinning”, or creating a void space, i.e., “void blowing”. Both are undesired effects which will adversely impact the flow of catalyst. These two undesirable effects are exacerbated by increasing the velocity of gas or throughput. Increasing the velocity or throughput is desirable because it permits an increase in capacity with only minor operating changes to the existing equipment. In addition, as the velocity of gas or throughput are increased, the pressure drop through the catalyst bed is also increased. The dehydrogenation of hydrocarbons is an equilibrium reaction, thermodynamically favored by high temperature and low pressure. Any increase in pressure causes a decrease in conversion and is undesirable.
Accordingly, there is a need for a process which reduces the pressure drop through the catalyst bed and increases the capacity of the reactors.